Characteristics of Carboniferous Lithofacies and Paleogeography in Eastern Part of North Qilian

The studied area is located in the eastern part of the North Qilian Caledonian foldbelt where the Carboniferous system with rich paleontological fossils develops perfectly and outcrops well.and the studies of the biostratigraphy have a long history. The two-fold division scheme for Carboniferous is adopted in this paper,i. e.,the Lower Carboniferous Series is composed of Qianheishan,Chouniukou and Tsingyuan Formations;and Upper one consists of Hong-tuwa,Yanghukou and Taiyuan Formations. The Qianheishan formation is equal to Tournaisian,Chouniukou to Visean,Tsingyuan and Hongtuwa to Namurian,Yanghukou and Taiyuan to Westphalian and Stephanian respectively. The evolution of the Carboniferous lithofacics and palcogography is described as follows: 1. This area was an epicontinental sea characterized by gulf. To the north was old Alaxa land;to the South Longxi uplift of Ordos. 2. During the Qianheishan-Tsingyuan period,the marine water invaded the area from Qinling through the Tianshui-Jingning strait,mak ing lion of this area became progressive it a gulf in the shape of“ overlap regulary. resultin;in the Hongtuw8-Westphalian,the invan developing a“Stephanian,the sea water expanded from North China into North Qilian,and conjoined Tianshan seas respectively shaped”gulf. In the with South China and 3. The coal-bearing alternative marine-continental facies of barrier coast consists mainly of elastic rocks and the region is composed of the plain facics zones of delta and seashore. The Lower Carboniferous contains gypsum. Although the fauna realm of the Visean,Namurian and Westphalian is similar to that of Euramerian,the sedimentary association is of the Qilian type. Taiyuan Formation belongs to the North China type charactcriccd by the Early Cathaysian flora. 4. Based on both the paleomagnetism and the paleoecology,this area was situated in the low latitudal zone in the Carboniferous period. It changed from the dry-hot in Tournaisian into damp-hot climate in the rest of the Carboniferous period. The Stcphanian stage is a main coal-forming period in this area. This area,the northern margin of Gansu-Qinghai-Tibet plate,acted as an ocean during the Early Paleozoic Era. The ancient oceanic crust should be subducted beneath the Center Qilian,and then,the collision between the Alaxa-Tarim and Caidam took place in the Caledonian,finally,resulting in the forming of the Hcxi-Corridor transitional }onc and the Caledonian foldbelt of the North Qilian. After that,the historical geology of Carboniferous in this area began to develop in the continental crust.